Polyarylate is typically an aromatic polyester composed of bisphenol A that is aromatic alcohol, terephthalate and isophthalate that is aromatic acid halides. The conventional polyarylate films have high degree of optical transmission, and are excellent in thermal and mechanical properties. However, the conventional polyarylate films are disadvantageous in that the melting temperature and viscosity is high, and the positive birefringence occurs in the surface direction during processing. As a result, in order to use polyarylates as optical films, many studies have been carried out on the method of controlling the birefringence in the surface direction.
Meanwhile, a display device using liquid crystal is disadvantageous in that a viewing angle is narrow due to intrinsic optical characteristics of a liquid crystal molecule and a polarizer. Particularly, in accordance with the recent trend toward enlargement of a liquid crystal display, the use of compensation films is required to assure a wide viewing angle. To enlarge a narrow viewing angle in the liquid crystal display, a material having a characteristic of delaying a phase difference of light is used. A delaying direction of the phase difference of light is classified into a surface direction and a thickness direction of the film. A material having birefringences to the directions is used in order to compensate the viewing angle. The phase difference of the compensation film is defined by the following equation (1):
                              R          th                =                              (                                          n                z                            -                                                                    n                    x                                    +                                      n                    y                                                  2                                      )                    ×          d                                    (        1        )            
in the equation (1), Rth is a phase difference in thickness direction, nx and ny are refractive indexes of the film in surface direction, nz is a refractive index of the film in thickness direction, and d is a thickness of the film. If there is no birefringence of the film in surface direction (nx=ny), when nx is more than nz, Rth has a negative value and, when nx is smaller than nz, Rth has a positive value.
Polymer chains must be oriented perpendicularly to the film surface or in a surface direction so that the polymer film has birefringence. In the case of when it is difficult to completely satisfy the above-mentioned condition, at least specific portion of the polymer chains must be oriented. The degree of the orientation of the polymer chains depends on components constituting the polymer, the film thickness, a drying condition of a solvent, etc. For example, if the film thickness is reduced to a molecular level, since the orientation of the polymer is maximized on a surface of the film, it is possible to obtain very high birefringence. Since the liquid crystal molecules have the positive birefringence, the material having the negative birefringence must be used to compensate the positive birefringence. A representative polymer having the negative birefringence is polystyrene. Further, if the film is uniaxially or biaxially stretched, an optic axis is oriented in a surface direction of the film. Accordingly, it is possible to obtain some negative birefringence even if it is not sufficient.
Generally, it is known that the birefringence in surface direction is used to produce a A-type compensation film. The birefringence in surface direction is obtained by stretching the polymer film in the surface direction to orient the polymer chains in the surface direction. On the other hand, the birefringence in thickness direction that is used to produce a C-type compensation film is obtained by uniaxially or biaxially stretching a film that is subjected to extrusion or solution casting. For example, a phase difference film in thickness direction for assuring a wide viewing angle is prepared by uniaxially or biaxially stretching a cellulose- or polycarbonate-based polymer film or by applying the liquid crystal molecules on a polymer film base substrate. However, since the birefringence in thickness direction capable of being obtained using the stretching is very little and changes a refractive index in surface direction, it is difficult to control the phase difference with respect to all directions. Additionally, in the case of when the film is greatly stretched at low temperatures in order to obtain high birefringence, the thickness of the film is reduced. Accordingly, there are disadvantages in that it is difficult to obtain the desired phase difference and the birefringence is nonuniform.
Furthermore, in the case of when polyarylate is applied on a base substrate such as polycarbonate, adhesion force between the base substrate and polyarylate may be poor. U.S. Pat. Nos. 6,100,367 and 6,174,966 describe a method of adding monomers, which are capable of providing an alkoxysilyl group, to polycarbonate or polyarylate during synthesis of polycarbonate or polyarylate so as to increase adhesion force to other base substrates. Similarly, U.S. Pat. No. 5,258,483 describes the use of polyarylates to which an epoxy group is added instead of an alkoxysilyl group.
Meanwhile, EP No. 1469328 discloses that a transparent optical film of polyimide is applied on a TAC (triacetyl cellulose) film to obtain uniform compensation and to remove irregularity causing rainbow-colored rays. JP-A-2001-194668 discloses a compensation film that is prepared by layering stretched polycarbonate films. However, the production requires a complicated layering process and also requires optic axes which are perpendicular to each other when two films are layered.
U.S. Pat. No. 5,043,413 discloses a method of preparing polyarylate having low birefringence in surface direction. In detail, in the above-mentioned US patent, polyarylate has low birefringence of 25.7×10−5 or less when the polyarylate is subjected to a solution casting process to produce a film, and then the film is stretched. However, the birefringence that is obtained using the stretching is the birefringence in surface direction, and is not useful for a C-type compensation film requiring the birefringence in thickness direction.
U.S. Pat. No. 5,285,303 discloses a method of uniaxially stretching a polyarylate film as a compensation film for a wide viewing angle and performing shrinkage in a direction perpendicular to the stretching direction so as to obtain the birefringence in thickness direction. A phase difference of liquid crystal is typically 100 to 400. In order to compensate the phase difference, it is necessary to assure a phase difference of 100 to 400 which has a value opposite to the former phase difference. However, the stretching is disadvantageous in that the film thickness is reduced and it is difficult to orient the polymers to great degree, thus there is a limit in assuring the desired phase difference.